We published a number of references in order to arrive at a more precise definition of open knowledge and its various aspects, see the entry here, inspired by the work of the Open Knowledge Foundation.
Here is a valuable refining by ben vershbow in the Future of the Book blog, on the occasion of the MIT conference on The Economics of Open Content:

“I heard that there are two kinds of “open.” Open as in open access — to knowledge, archives, medical information etc. (like Public Library of Science or Project Gutenberg). And open as in open process — work that is out in the open, open to input, even open-ended (like Linux, Wikipedia or our experiment with MItch Stephens, Without Gods).

I heard that “content” is actually a demeaning term, treating works of authorship as filler for slots — a commodity as opposed to a public good.

I heard that open content is not necessarily the same as free content. Both can be part of a business model, but the defining difference is control — open content is often still controlled content.

I heard that for “open” to win real user investment that will feedback innovation and even result in profit, it has to be really open, not sort of open. Otherwise “open” will always be a burden.

I heard that if you build the open-access resources and demonstrate their value, the money will come later.

I heard that content should be given away for free and that the money is to be made talking about the content.

I heard that reputation and an audience are the most valuable currency anyway.”